This invention relates to control devices and more particularly to a pneumatic control unit for selectively directing the operating pressure, cycle and work function duration of a pneumatic work device. Pneumatic work devices, most commonly air actuated pistons, find many light to moderate industrial applications, such as in the holding, moving, indexing, piercing and pressing of a work piece or in the indexing and actuation of the tools and fixtures or machines which perform manufacturing functions on the work piece itself.
It is common in such industrial applications, to perform various manufacturing operations on a given work piece or on a given machine in a timed, cyclic manner. In automatic or semi-automatic machines it is essential that the workpiece be precisely indexed to its proper work station at the proper time and that any machine functions operating on the work piece at a given work station such as hold downs, air blasts, tools or the like be also precisely cycled to perform their intended tasks for a prescribed duration of time.
With this in mind, it it thus necessary to utilize control devices which are either timed or which physically sense the movement of the work piece, machine, tool or the like so as to achieve satisfactory operation. Heretofore, it has been common to use air actuated pistons for various industrial functions, as alluded to above, and to control the cycling and timing or duration of their work functions by use of electrically actuated limit switches and timers which open and close electric solenoid valves to control the air flow in the given system. These prior control systems have certain inherent shortcomings since they rely upon mechanically accuated limit switches which may malfunction due to fouling, misadjustment or machine vibration. In addition, these devices require electricity and, in certain plant locations, this may prove burdensome and expensive to wire or even dangerous, if there is a fire or explosion danger, due to electrical sparking, in a given location.
In order to overcome these shortcomings, it has been recognized in the past that it is desirable, when and where possible, to do away with the aforementioned electrical limit switches and solenoid valves and, in their place, utilize pneumatic control devices exclusively.
In an effort to meet this need, as well as others, the pneumatic control industry has developed numerous types and sizes of air flow control valves which are designed to perform various functions such as automatic cycling and timing in connection with pneumatic work systems.
For example, it has heretofore been recognized that a pair of three-way, time delay valves when piped in a series pneumatic circuit and directly piped to a spring loaded work piston could function as an automatic cycler. It has further been recognized that the duration of the cycle of the work piston could be varied by adjusting the timing controls on the three-way valves and that the time delay could be further increased by the addition of volume chambers to the circuit.
Another known pneumatic cycling arrangement utilizes two three-way valves, without the time delay feature, piped in a parallel pneumatic circuit to a double piloted, four-way valve. The air output from the four-way valve is piped to a double acting work cylinder with a reciprocating piston. This circuit permits reciprocal movement of the piston without the use of limit switches or valves and controls the piston speed by restricting the air exhausting from the four-way valve.
While these aforementioned, known pneumatic circuits do produce an automatic cycling function, they also have some inherent shortcomings. For example, in the series pneumatic circuit arrangement, first mentioned above, one of the three way time delay valves is piped directly to the work device and the work device thus becomes part of the pneumatic circuit. The work device is a single acting cylinder having a spring loaded piston to achieve retraction thereof so as to permit the work cylinder to exhaust through the three-way valve. Hence, the cycling of the work device is dependent to a great extent upon the spring return of the work cylinder's piston which could pose problems where true cycle duration and constant length of extension and retraction of the piston stroke are necessary. In addition, this prior circuit has only one air output to the work device which, necessarily, limits its usefulness.
The known parallel pneumatic circuit mentioned above, likewise, lacks certain desirable features which renders it somewhat deficient in situations where an adjustable range of cycle times is sought. In this prior circuit, the three-way valves have no adjustable time delay knobs to enable the user to pre-select the timing in actuating of the four-way valve. Time delay is merely pre-set at one valve, for a given air line pressure, by the size of the restricted exhaust orifices in the four-way valve. Further, this prior known parallel circuit makes no use of auxiliary volume chambers which further reduces the potential for achieving longer cycle times.
In addition to the several deficiencies noted above relating to the functioning of prior, known parallel and series pneumatic cycling circuits, there are also other shortcomings present in the prior art envolving the layout and use of pneumatic controls and circuits in general. In many instances, an industrial user of pneumatic devices designs and builds his own pneumatic control system. In most cases, the pneumatic control devices, valves, regulators, and gauges are mounted on a board or table or along in-plant air lines, uncovered and sometimes distantly spaced from other pneumatic circuit components. This common layout practice envolving pneumatic control circuits not only leads to awkward timing and fine tuning of the system, but it also may lead to premature component failure because the parts are physically exposed to the plant environment and could be accidentally damaged by workers or generally degraded by dirt and dust present in the plant.